Since 1990 some 230 tunnels have been found. How many have been missed?

Presumably these are mostly in urban areas where it might make economic sense to go underneath and where existing drainage tunnels exist. Also assume this is more CA, AZ and NM since the US/Mexico has a river.

Border Patrol agents have discovered at least 230 cross-border tunnels since 1990

Debate over funding for the border wall comes amid growing questions about a barrier's effectiveness in the fight against drug trafficking

www.cbsnews.com